Sendmail is a Mail Transfer Agent, which is the
program that moves mail from one machine to
another. Sendmail implements a general
internetwork mail routing facility, featuring
aliasing and forwarding, automatic routing to
network gateways, and flexible configuration.

Bulk Mailer helps speed the delivery of an email message to many recipients using Sendmail. It's best used in parallel with a mailing list manager such as Majordomo. For input, it takes a file of recipient addresses (one address per line) and a message (with headers already attached) to be sent to the recipients. It sorts the recipient list by reversed domain (so similar ones sort together), splits up the recipients into several groups containing no more than N domains each, creates an SMTP envelope for each group of recipients, and feeds each envelope to sendmail.

MiniMTA is a simple class that acts as a tiny MTA to relay
messages via SMTP. The message is taken from standard input, and
transmitted to the server given on the command line. It is
based loosely on nbsmtp.

mail_body is an SMS email forwarder, written in PHP4. It splits the body of
incoming email into 160-character long chunks, and sends them to a mobile phone
email address. See the source code comments for instructions.

PHP Mobile Mail is aimed at mobile professionals
who want to use their favorite MUAs (noteably pine
or mutt) on their laptop, either in UNIX or Windows
(under GNU Gygwin). This lets you work offline. It
includes a mail_spooler program, which accepts the
output of you MUA and spools (on, or offline).
When connected, another process parses through
the spool and sends your messages. It is also
designed to "fake" using your REAL email address
with free services like gmx.net, or mail.yahoo.com,
when using their POP and SMTP servers, assuming
you have a domain and account that you can
control to this degree.

QwikMail is a mail server solution that includes an SMTP server that
can function as a full MTA, as well as a local delivery agent that can
deliver to both /var/spool mbox files as well as Maildirs. It includes
RBL checking at the SMTP stage.